Electronic scanning apparatus



R A. WESTERVELT ETAL 2,645,741

ELECTRONIC SCANNING APPARATUS July 14, 1953 5 Sheets-$heet 1 Filed March 29, 1948 FIG.

INVENTORS ROBERT A. WESTERVELT ROBERT L. CUMMEROW ATTORNEY July 14, 1953 R. A. WESTERVELT EI'AL ELECTRONIC SCANNING APPARATUS 5 Sheets-Sheet 2 Filed March 29, 1948 INVENTORS ROBERT A. WESTERVELT ROBERT L. CUMME OW ATTORNEY y 1953 R. A. WESTERVELT ETAL 2,645,741

ELECTRONIC SCANNING APPARATUS 3 Sheets-Sheet 3 Filed March 29, 1948 INVENTORS' ROBERT A; WESTERVELT ROBERT L. CUMMEROW AT TORNE Y Patented July 14, 1953 ELECTRONIC SCANNING APPARATUS Robert Alanson Westervelt, Ames, Iowa, and Robert L. Cummcrow, Schenectady, N. Y.

Application March 29, 1948, Serial No. 17,604

(Granted under Title 35, U. S. Code (1952),

sec. 266) 15 Claims.

The present invention relates broadly to electrical apparatus and systems, and more particularly, it relates to the art including electronic switching devices. The invention has for an object to provide a generally improved electronic switching arrangement capable of actuating selected ones of a relatively large number of electric circuits.

Systems are known wherein a relatively large number of electric signals are carried in individual channels, and wherein the latter are desirably actuated cyclically, or in any other sequential manner, to eiiectuate a desired mode of operation.

For example, object-detecting systems adapted for underwater operation utilize transducers having arrays of electro-mechanical elements of the piezoelectric or magnetostrictive type. Such elemental arrays cooperate with suitable lag or lead lines to determine directivity characteristics of the transducer, which define the power of the transducer to pick up and/or transmit compressional wave energy more effectively in a direction corresponding to its socalled directivity maximum and considerably less efficiently in other directions. Further, such systems are rendered capable of scanning or rotating the directivity maximum by means of arrangements for switching the connections between the transducer elements and the lag or lead lines associated therewith.

Heretofore, various mechanical and/or electrical switching arrangements have been proposed for accomplishing the switching function, but such arrangements as have been proposed are either excessively bulky or unwieldy, rendering the apparatus incapable of being operated at required high speeds of rotation, or the priorknown arrangements contemplate the use of such large numbers of electron discharge tubes, each of which are required to have substantially identical characteristics, as to render such arrangements practically impossible of attainment.

Accordingly, it is another object of the invention to provide a novel and improved switching arrangement that obviates the disadvantages of known arrangements in that it embodies no moving parts other than electrons moving under the influence of electric and magnetic fields, said system also being capable of actuating a relatively large number of individual circuits without requiring a correspondingly large number of electron tubes.

Another object is to provide an electronic switching device for actuatin a plurality of circuits seriatim, said device including a multi anode electron discharge tube of which the num ber of anodes corresponds to the number of cir cuits to be actuated, and means establishing conductive path to individual anodes.

Still another object is to provide an improve| electronic switching arrangement wherein a uni tary electron discharge tube is arranged with common source of electrons disposed axially ii the tube, a plurality of longitudinally spacer anode rings arranged around the common some and radially spaced therefrom, and means cycli cally establishing a conductive path between in dividual ones of the anode rings and the commoi source of electrons.

A further object is to provide an improved ob ject-detecting system in which scanning is ac complished by means including a transduce having a plurality of pressure-responsive e e ments adapted to develop electric signals in ac cordance with wave energy incident thereon, delay line for introducing predetermined amount of time lag between signals from individual ele ments, and an electronic switching tube f0 switching the connections between the individua elements of a transducer and the terminals of delay line.

Other objects and advantages will become ap parent from the following specification taken i1 connection with the accompanying drawin; wherein the invention is embodied in concret form.

In the drawing:

Fig. 1 is a block diagram, partly diagrammati and partly schematic, of an object-detecting sys tem incorporating the features of the inventior Fig. 2 is a diagram representing an operatin; characteristic of the system of Fig. 1,

Fig. 3 is a side view, mainly in longitudinal sec tion and partly schematic, of an electroni switching device constructed according to th present invention,

Fig. 4 is a cross-sectional view taken along th line 4-4 of Fig. 3,

Fig. 5 is a fragmentary diagrammatic view il lustrating a skewed electronic lamina and radia beam arrangement that is developed and utilize in the tube of Fig. 3,

Fig. 6 is a fragmentary developed view of on form of grid structure that may be employed i: the tube illustrated in Fig. 3, and

Fig. 7 is a fragmentary developed view of 1 modified form of grid structure that may b utilized with the tube of Fig. 3.

like reference characters are utilized throughthe drawing to designate like parts. ig. 1 illustrates the listening branch of a sublic, sonic or supersonic object-detecting sys- 1 embodying the features of the present inltion. It should be understood clearly, howr, that, although the invention is herein deibed in connection with a compressional-wave ect-detecting system, it is by no means limited :reto, it being readily applicable to other sysis as, for example, radar, and/or other com- ,nications systems, electronic calculating dees, and similar electrical arrangements wherea relatively large number of signals carried in ividual channels are selectively utilized to perm a stated function. ls shown in Fig. 1, the listening apparatus nprises a hydrophone or transducer diaimmatically illustrated, and which may be of v conventional type having a plurality of pres- 'e-responsive elements |3 suitably mounted and 'anged in cylindrical array within a substanlly cylindrical water-proof container or boot adapted to be submerged and fixedly supported ieath the surface of the water with its struc- 'al axis vertical. is is well known, compressional wave energy iking the active face of the boot |5, is transtted therethrough to the elements l3 causing 2 latter to generate alternating-current signals magnitude and frequency corresponding to a amplitude and frequency, respectively, of the :ident wave energy. [*he signals from the transducer elements l3 2 fed, by means of a multi-conductor cable through a switching device hereinafter to described, and thence to a delay network l9, .ich is in the form of a multi-unit multi-ternal network of inductances L and capacitances Terminals l4 are provided at the junction adjacent units, to individual ones of which *minals are connected (not shown) pairs of iductors from corresponding transducer eleznts |3 disposed on opposite sides of a predemined reference of direction N5 of transducer In this manner, predetermined amounts of ase shift are introduced among the signals deed from said pairs of elements l3, resulting the production of a receiving sensitivity maxiim that is colinear with the reference l6 of the LIlSdllCEI A typical receiving sensitivity or directivity tximum is illustrated by the curve 2| in Fig. :howing a maximum response characteristic at int 23 and supernumerary side lobes 25, 21. i6 side lobes are relatively unimportant for a present discussion and will not be described rther. The amplitude of the directivity pat- 'n, however, is of interest, and its magnitude in ,ation to the general shape of the curve 2| is aful in determining the directional characterics of the transducer, which in turn, determine a usefulness of the transducer and the listen- 3 system as a search instrument. By cyclically switching the connections beeen the elements l3 of the transducer II and 2 terminals l4 of the delay network I9, the 'ectivity pattern is caused to rotate or scan rough any desired angle in azimuth or elevan. After suitable amplification of the switched 'nals in a conventional amplifier 29, visual in- :ations of energy received during scanning of e directivity pattern may be provided by a suitle indicator 3!! so that objects that lie within age of the listening branch and, which either themselves emit, or permit wave energy of suitable frequencies to be reflected therefrom, may readily be located as the scanning operation proceeds.

A switching device I embodying the principles of the present invention is shown in Figs. 3 and 4, consists in an electron-discharge tube having substantially cylindrical envelope 3|, which may be of glass, and a base assembly 33. Base assembly 33 may be of any conventional design and type such as are used for vacuum tubes, generally, for anchoring support rods, and through which conductors may be drawn connecting electrodes within the envelope 3| and pins or terminals 35 mounted exteriorly of the base assembly. To this end, base 33 may be formed of some moldable insulation material 3'! such as a phenolic condensation product contained within a metallic cup 39, the open end of which is feathered as at 4| for bonding to the adjacent end of the envelope 3| in a conventional air-tight seal so as to maintain a vacuum within the envelope 3 I.

For providing a copious supply of electrons within the envelope, a cathode or electron emitter unit comprising an elongated hollow tube 43, of which the exterior surface may be coated with an electron-emissive material is mounted coaxially of the envelope 3| and is supported at its opposite ends by means of rods 45 and 41. At one end, the rods 45 are embedded in the wall of envelope 3| and at the other, the rods 45 are fixed, as by spot-welding, to a disk 49 to which the adjacent end of tube 43 is also securely afiixed. The rods 41 are spot-welded to a ring 5| to which the opposite end of tube 43 is fixed, while the opposite ends of rod 41 are anchored in the insulation material 31. One of the rods 4'! is preferably connected to pin 53 through which it may be connected to ground, as at 55.

A heater filament 51 is mounted within the cathode tube 43 and is adapted, upon energization by a current from a battery or other suitable potential supply 59, to cause electrons to be emitted from the surface of tube 43.

Radially spaced from the cathode 43 and supported in coaxial alignment therewith is a beamsuppressing grid assembly 60 comprising a lon gitudinal array of segmented rings 6|, each said ring being formed of three equal-length arcuate conductive segments 63, 65, and 61 spaced by substantially smaller insulating segments 69. The segmented rings 6| are preferably uniformly spaced axially along the tube and supported on three conductive rods or supports 1|, l3, and 15, to which the sections 63, 65 and 67, respectively, are suitably connected, as by spot welding or silver soldering, with the rods H, l3, '15 disposed at the mid-points of the respective sections 63, 65, 61, as shown in Fig .4.

For supporting the upper ends of the rods 13, and 15, an insulating ring 11 is mounted so that the cathode 43 passes through a central aperture therein, the ring ll being rigidly positioned and held by means of radial struts 8| molded or embedded in the envelope 3|.

The lower ends of the rods 7|, l3 and are brought out through the base 33 to pins 83, and 81, for connection to a three-phase electrical generator 9|. An adjustable phase-control device 89 is connected in the energization line to the beam-suppressing grid 69 for purposes that will presently appear.

A signal grid assembly 93 is coaxially disposed within the envelope 3| and surrounds the suppressor grid 60 and cathode 43 just described. The signal grid. comprises a squirrel-cage formed of longitudinal wires or rods 95 the lower ends of which are embedded in an insulating ring 91, which, in turn, is supported from the base 33 by means of studs 99. The upper ends of the signal grid wires 95 are brought out, through suitable openings in the insulating ring TI and through vacuum-tight seals in the envelope 3|, to an enlarged insulating ring |I to provide input terminals I03 for connection to the conductors of cable I2 (Fig. l) and thence to the pressure-responsive elements I3 of transducer II.

While it may appear from Fig. 3 that the individual wires or rods forming the signal grid are of diameter of the same order of magnitude as the spacing between adjacent wires or rods, it will be understood that the showing is exaggerated, and that commonly, the diameters of the signal grid wires or rods are substantially less than the spacing therebetween.

Anodic ring electrodes in the form of cylindrical metallic plates or rings I 05 are longitudinally spaced and arrayed coaxially with the cathode, suppressor grid and signal grid assemblies, above described. The rings I05 carried by insulating standards I0! which, in turn, are suitably anchored in the base 33. The rings I95 may be secured to the standards in any suitable manner, and carry output leads I09, which are passed through vacuum-tight seals in the envelope 3| to enable connection to the terminals IA of the delay network I9 (Fig. 1).

Shielding electrodes in the form of cylindrical metallic plates or rings III, generally similar to the anode rings I05, are also axially spaced from each other and coaxially mounted within the tube [1 on conductive rod supports I I3. Supports I I3 are anchored in the base 33, and one such support is brought out to a pin H5 for conductive connection to the positive terminal of the battery 59. Rings III are diametrally dimensioned for disposition between the signal grid assembly 93 and the anode rings I05, and are preferably interpolated between adjacent anode rings I05 so as to shield the signal grid wires 95 from the output rings I05, and, at the same time, to define annular gaps through which radially extending electron beams may pass. Rings I I I also serve to reduce cross-talk between any single output ring I05 and those adjacent thereto.

The broken-line rectangle I I1, drawn to enclose a length of the tube I'I substantially corresponding to the active portion of the cathode, designates any suitable conventional apparatus for providing a rotating magnetic field, which field is preferably directed radially of and perpendicularly to the axis of tube l I, and adapted to rotate about an axis colinear with the tube axis. A suitable apparatus for generating such a rotating magnetic field is the stator of a three-phase motorconnected to be driven from the threephase generator 9i.

The following description 01 the operation of our switching tube is made with reference to a transducer having 48 elements, such as are in common use in scanning sonar systems. It should be understood distinctly, however, that the selection of a sonar system and/or the selection of the specified number of transducer elements are merely exemplary, and are not to be construed as limitations of scope.

In operation with such a 48-element transducer, it is generally desirable that the switching device simultaneously control 18 of the 48 elements. That is to say, as the switching operation proceeds, it is desired that circuits including 18 of the transducer elements be mad and the circuits including 18 elements be broken all within a given time interval. Such operatiol is accomplished in the tube IT, by rotating a plu rality of radially extending beams of electron that are formed from a rotating electron shee or lamina, which, in turn, is derived by the join action, on the electron cloud from cathode 43 of the rotating magnetic field generated by th apparatus I I1 and the rotating bias on the beam suppressing grid assembly 60. The steps in th formation of the plural radial beams are now t be considered.

The electrons from the cathode 43 are emitter substantially uniformly along the length of th cathode and around the periphery thereof, form ing a substantially cylindrical electron cloud The cloud is transformed into a sheet-like o laminar distribution of electrons by the action 0 the radial magnetic field in accordance with well known theories of electron ballistics. The lam ina of electrons is normally planar and extend diametrally across the tube I! with the plane 0 the lamina including the axis of the tube. Th lamina, of course, is caused to rotate about th axis of the tube by and in accordance with r0 tation of the radial magnetic field.

In the form of the invention illustrated in Fi 3, characterized by the fact that the signal gri wires are arranged parallel to the axis of th tube H, to accomplish sequential switching, it i necessary that the plane of the rotating lamin be skewed at a pitch such that, for a given orien tation of the lamina, it is modulated by signal from a predetermined number, which in the as sumed example is 18, consecutive transducer ele ments. Such a skewed lamina is shown at II in Fig. 5, and is derived by skewing the windin slots and the windings of the polyphase moto stator III.

As described above, the suppressor grid as sembly 60 (Fig. 3) is energized from the sam polyphase generator 9| as is the magnetic-field producing apparatus II I. Accordingly, it wij be seen that the alternating potential on the seg ments 63, 65 and 61 of the suppressor grid i polyphase and varies at the same frequency a the rate of rotation of the magnetic field pro duced by apparatus III. Thus the half of th electronic lamina I I9 (in Fig. 5) that is directe toward the negatively energized segments of th suppressor grid is effectively suppressed, whil the half H8 directed awa from the negativel energized segments is radially accelerated an rotated by the rotating field.

To ensure smooth and continuous rotation c the non-suppressed half of the lamina H8, th adjustable phase-control device 89 (Fig. 3) is (16 sirably inserted in the lines from the generatc 9i to the suppressor grid 60 to adjust and con trol the relative phases of the polyphase energ used to produce the rotating field and that use to energize the grid 69. The relative phases ar adjusted so that peak voltages are developed 0 the grid segments slightly in advance of the volt age peaks developed in the magnetic apparatu whereby the arrival of the rotating lamina occu. slightly after peak positive voltage is develope at the corresponding grid segment.

The rotating lamina interacts with the sign: grid wires 95, of which there are 48 in the illus trative example herein assumed, each of whic receives a signal voltage from a correspondin transducer element I3 and modulates the lamin therewith. The signal modulated lamina :reupon transformed into radial beams 12! g. by the shielding rings Ill. The beams pass through the annular gaps between the gs Ill and impinge on the 18 anode rings H35, ence output signals corresponding to the modtion signal of the beams are delivered to the ay network [9. is above described, the network is combines signals interposing therebetween suitable ounts of delay, and thus causes a sharp sensi- .ty maximum to be defined, of which the dition of maximum sensitivity is indicated by e reference of direction [6 (Fig. 1). ig. 6 diagrammatically represents the spatial entation of the signal grids 95 and anode rings i in relation to the skewed electron lamina and beams IZI. As shown, the rings m5 1 grids 95 form a substantially rectangular tice that is intersected by the beams l2! and iina H8 of electrons in a region here shown the dotted lines I23 oriented skew-wise there-- we The dotted lines I25 represent the insection of the lamina and beams after rotan thereof through an angle of 7 /2 degrees. It 1 be noted that in the position of the beams 11 lamina at I23, signals from 18 grid wires .ignated serially c through t modulate the ms 12 I, and the beams, so modulated, impinge the anode rings I85. After rotation of the HHS through 7 /2 degrees, signals from grids b 'ough s modulate the beams and the latter, as y modulated, impinge on the anode rings H15. t will be seen that, as the lamina H8 and e electron beams l2! rotate through the preermined angle, in the illustration herein asned, equal to 7 /2 degrees, signals from sucsive arrays of 18 consecutive elements are ised to modulate the beams, the modulated ims impinge on respective anodes I05, and the put circuits I09 connected to the anodes I05 eive and carry signals characteristic of arrays elements successively spaced 7 /2 degrees from a preceding arrays. Thus, as the lamina H8 1 beams l2! continue in rotation, the senitivity ximum 2| is formed from signals successively ferent arrays of elements and, accordingly, lgresses around the transducer ll.

F0 ensure proper synchronization of the voltas applied to the segments 63, 65 and 61 and a rotary movement of the skewed beams I21, arrangement of the segments may also be :wed at substantially the same pitch as the :wing of the beams. However, even without kewed arrangement of the segments 63, 65 and deviation from perfect synchronism is not so .rked as to interfere with satisfactory funcning of the tube.

ig. 7 illustrates an alternative construction of nal grids that may be employed in the switchtube. As shown, wires l2'i, forming the siggrid, are disposed in substantially helical hion. With the grid wires so arranged, an ctron lamina I29 and beams [3| are formed 1 disposed in a vertical plane parallel to the s of the tube, thus obviating the necessity of :wing the Winding slots and windings of the .gnetic-field producing apparatus Ill. The ch of the grid wires is adjusted, as was the ch of the beam in the embodiment of Figs. i, so that for a given angular position of the mm, the latter is modulated by the signals from predetermined number of consecutive trans- :er elements.

in all other respects, a tube constructed with nal grid and beam arranged in accordance with the illustration of Fig. '7 is substantially similar in structure and in operation to that described hereinabove.

There has thus been described a scanning sonar system embodying an electron discharge tube that is constructed to perform the necessary switching operations to supply proper signal combinations from an array of pressure-responsive elements to a stationary beam-forming network.

It will be seen, that the tube is, in effect, an electrical analogue of a mechanical commutator with multiple slip rings and brush connectors. By the presently described device, the rotating mass of a mechanical commutator is advantageously replaced with a plurality of rotating beams of electrons thereby rendering this commutator susceptible to operation at considerably greater rotative speeds, and hence higher rates of scanning without being subject to the manifold mechanical design problems of highspeed commutators. Furthermore, the frequency of rotation of the beams, and hence the switching rate, may readily be adjusted as by varying the frequency of the polyphase energy from source 9|, so that variable scanning speeds are thus provided.

While the switching or commutator tube has hereinabove been described as embodied in a sonar system, it will be understood that it is readily adapted for use in many other systems such as, for example, in radar, radio-direction finding, instrument landing and other high-speed scanning systems. It will be noted-further, that any desired number of circuits, other than the herein specified number, may conveniently be serially switched.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above descripion or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes Without the payment of any royalties thereon or therefor.

We claim:

1. Switching apparatus comprising an envelope, means within said envelope for providing supply of electrons, means for forming said electrons into a lamina and for rotating said lamina, a plurality of circuits to be switched, grid means in the path of said rotating lamina and adapted for energization by electric signals from said plurality of circuits to modulate the rotating lamina, and a plurality of anodes spaced from said grids and arranged selectively to receive portions of said rotating electronic lamina after modulation thereof, whereby signals from individual circuits appear in succession on individual anodes.

2. Switching apparatus comprising an evacuated envelope, means Within said envelope for generating a supply of electrons, means for forming said electrons into a lamina and for rotating said lamina about an axis extending through said envelope, a plurality of grids in the path of said rotating lamina and adapted to be energized by electric signals from a like plurality of circuits to be switched, said grids being angularly disposed relative to said lamina thereby to modulate the rotating lamina with signals from individual circuits in succession, and a plurality of anodes spaced from said grids and arranged selectively to receive portions of said rotating electronic lamina after modulation thereof by the energized grids, whereby signals from individual circuits appear in succession on individual anodes.

3. Electronic switching apparatus, comprising an evacuated envelope, cathode means axially mounted within said envelope and adapted to provide a substantially cylindrical cloud of electrons, means for forming said electron cloud into a lamina and for rotating said lamina at a predetermined speed, means for transforming the peripheral part of said lamina into a plurality of radial beams, a plurality of grid elements in the path of said rotating lamina and adapted for energization by electric signals from a like plurality of circuits to be switched to modulate said radial beams, and a. plurality of anodes spaced from said grids and arranged selectively to receive said radial beams after modulation thereof by the energized grids, whereby signals from individual circuits appear in succession on individual anodes.

4. Electron-discharge tube apparatus, comprising means providing a cloud of electrons, means for forming said electron cloud into skewed electronic lamina having radially extending beam portions and for rotating said lamina and said beam portions about an axis in said tube, grid means axially disposed in the path of said rotating lamina and adapted, upon energization thereof by electric signals from a plurality of circuits to be switched, to modulate the rotating beam portions, a plurality of anodes spaced from said grids and adapted selectively to receive said beam portions after modulation thereof by the energized grids, and means interposed between said grid means and said anodes for minimizing crosstalk between adjacent anodes.

5. An electron discharge tube, comprising means for producing supply of electrons, means for forming said electrons into a plurality of beams mutually spaced along a common axis in the tube, said beams extending along substantially parallel planes, magnetic-field-producing means for simultaneously rotating said beams about said axis a plurality of signal grids in the path of said rotating beams, each said grid bearing an electric signal characteristic of a condition and adapted to modulate the beams thereby, a plurality of anodes surrounding said grids, individual ones of said anodes being adapted to receive a respective one of said beams, and means controlling the rotative motion of said beams, whereby said individual anodes successively receive signals from respective grids.

6. The electron discharge tube defined in claim wherein said signal grids are disposed in skewwise relation with respect to the axis of the tube, and the beams are coplanarly arranged.

7. The electron discharge tube defined in claim 5 wherein said signal grids are filamentary and disposed parallel to each other and to the axis of the tube and the beams are skewed.

8. An electron discharge tube, comprising means for producing a plurality of electron beams mutually spaced along a common axis in the tube, said beams extending radially of the tube and in substantially parallel planes, means for simultaneously rotating said beams about said axis, grid means in the path of said rotating beams, and a plurality of anodes surrounding said grid means with at least one grid for eacl anode, individual ones of said anodes being alinel with individual ones of said beams defining there with a plurality of conductive paths in the tube 9. The electron discharge tube as in claim further characterized by signal input terminal connected to individual grids and individual out put terminals connected to said anodes, whereb; electric signals from said grids are successivel conducted to said anodes.

10. An electron discharge tube, comprisin means for producing a supply of electrons, mean for forming said electrons into a plurality o beams mutually spaced along a common axis i1 said tube, said beams extending in substantiall a single direction, and means for rotating sai beams about said axis simultaneously.

11. An electronic distributor comprising at envelope, a cathode axially located thereir means for forming electrons from said cathod into a radial beam, modulation electrode mean comprising a plurality of filamentary conductor arranged parallel to each other and to the axi and in the path of said beam, and a plurality c anodes surrounding said modulation-electrod means.

12. Electron discharge apparatus comprisin an envelope, means within said envelope for pro viding supply of electrons, means for formin said electrons into a lamina and for rotating sai lamina, grid means in the path of said rotatin lamina and adapted to modulate the rotatin lamina, and a plurality of annular anodes space from said grid means and arranged in space concentric alignment selectively to receive por tions of said rotating lamina after modulatioi thereof.

13. The apparatus as in claim 12 wherein sai grid means comprises a cylindrical array of con ductors, the individual conductors being parallr to each other and inclined to the axis of sai envelope.

14. The apparatus as in claim 12 wherein sai grid means comprises a plurality of conductoi disposed in cylindrical array and in the path said beams, said conductors being mutually inst lated from each other and adapted to be indz vidually energized by signals from individual one of a plurality of circuits.

15. The apparatus as in claim 12 further com prising a plurality of shielding rings, individu: ones of said rings being interposed between adja cent pairs of said anodes and in partly overlar ping relation therewith.

ROBERT ALANSON WESTERVELT. ROBERT L. CUMMEROW.

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